1. Field of the Invention
The present invention relates in general to an oxygen sensing apparatus, in particular, an apparatus adapted to sense exhaust gases such as those emitted by internal combustion engines of a motor vehicle or various industrial furnaces, and more specifically to such an exhaust-gas sensing apparatus used for a combustion control system for the internal combustion engines and industrial furnaces, to determine an air/fuel (A/F) ratio of an air-fuel mixture supplied to the engines and furnaces. In particular, the invention relates to such an A/F-ratio sensor which has a function of compensating for its chronological changes in the operating characteristics and deterioration in the durability.
2. Discussion of the Prior Art
As an oxygen sensor for determining an oxygen concentration of combustion exhaust gases emitted for example by automotive internal combustion engines, there is known a sensor which utilizes a zirconia ceramic or other oxygen-ion conductive solid electrolyte material and which is operated to determine the oxygen concentration according to the principle of an oxygen concentration cell. For operating an internal combustion engine, it is required to accurately control an air/fuel (A/F) ratio of an air-fuel mixture supplied to the engine, such that the actual air/fuel ratio coincides with a desired value. Generally, this air/fuel ratio is determined by measuring the concentration of oxygen in the exhaust gases, which is varied as a function of the air/fuel ratio of the air-fuel mixture supplied to the engine. A signal representative of the determined air/fuel ratio is fed to a fuel supply control system of the engine, in order to determine an amount of supply of the fuel, i.e., to control the actual air/fuel ratio so as to coincide with the desired value.
An example of such an oxygen sensor (oxygen-concentration detector) used as an A/F ratio sensor is proposed in U.S. Pat. No. 4,568,443. In the oxygen sensor proposed in this publication, a sensing element of the sensor is formed with an internal gas-diffusion space which communicates with an external measurement-gas space in which there exist exhaust gases (hereinafter referred to as "measurement gas" when appropriate) to be measured. The sensing element has detecting means for detecting the oxygen concentration of the atmosphere within the internal gas-diffusion space, which consists of the measurement gas which is introduced under a predetermined diffusion resistance. The detecting means produces an output indicative of the oxygen concentration of the atmosphere within the gas-diffusion space. The sensing element also has oxygen pumping means which is operated with a pump current based on the output of the detecting means, so that the oxygen concentration within the diffusion-gas space is maintained at a predetermined level. The pump current applied to the oxygen pumping means is measured as a parameter which represents the oxygen concentration of the measurement gas, i.e., exhaust gases which are produced as a result of combustion of an air-fuel mixture. Namely, the pump current represents the air/fuel ratio of the air-fuel mixture.
Described more specifically referring to FIG. 2, the sensing element of the A/F-ratio sensor of the type discussed above includes a first electrochemical cell in the form of an oxygen pumping cell which comprises a solid electrolyte body 2, an inner pumping electrode 6 which is exposed to an internal flat gas-diffusion space 4 communicating with an external measurement-gas space, and an outer pumping electrode 8 which is substantially exposed to the external space. The sensing element further includes a second electrochemical cell (oxygen concentration cell) in the form of an oxygen sensing cell which comprises the solid electrolyte body 2, a measuring electrode 10 which is exposed to the internal flat gas-diffusion space 4, and a reference electrode 14 which is exposed to an air passage 12 communicating with the ambient air. The sensing element incorporates a heater 16 adapted to keep these two oxygen pumping and sensing cells at suitable operating temperatures. The oxygen sensor having this sensing element is capable of dealing with not only stoichiometric exhaust gases, but also lean-burned exahust gases and rich-burned exhaust gases. The stoichiometric exhaust gases are exhaust gases which are produced as a result of combustion of an air-fuel mixture whose A/F ratio is equal to or near the stochiometric value (A/F=14.6) or whose excess air factor (.lambda.) is equal to or near "1". The lean-burned exhaust gases are exhaust gases produced by combustion of an air-fuel mixture whose A/F ratio (excess air factor) is larger than the stoichiometric value ( .lambda.&gt;1), while the rich-burned exhaust gases are exhaust gases emitted by combustion of an air-fuel mixture whose A/F ratio (excess air factor) is smaller than the stoichiometric value ( .lambda.&lt;1). Thus, the instant oxygen sensor is capable of determining the A/F ratios of the air-fuel mixtures which give these different types of exhaust gases.
In the A/F-ratio sensor of the type described above, the measurement gas is introduced from the external measurement-gas space into the internal gas-diffusion space 4 under the predetermined diffusion resistance, so that the introduced measurement gas contacts the measuring electrode 10 of the oxygen sensing cell. In the meantime, an electric current, so-called "pump current" (Ip) is applied between the inner and outer pumping electrodes 6, 8 disposed within and outside the gas-diffusion space 4, so that a well known oxygen pumping action is performed such that the oxygen concentration of the atmosphere adjacent to the measuring electrode 10 is maintained at a predetermined level, due to the reaction of the pumping electrodes 6, 8. According to this arrangement, the pump current (Ip) is varied with a change in the concentration of a desired measurement component (i.e., oxygen concentration) of the measurement gas (exhaust gases), that is, with a variation in the excess air factor (.lambda.) or A/F ratio of the air-fuel mixture which gives the exhaust gases. Consequently, the measurement of the pump current (Ip) makes it possible to determine the excess air factor (.lambda.) of an air-fuel mixture which is higher or lower than "1", or the A/F ratio of the same which is higher or lower than the stoichiometric level. In other words, the instant A/F-ratio sensor is capable of dealing with not only the lean-burned exhaust gases which are produced as a result of an air-rich air-fuel mixture and which has a higher oxygen partial pressure than the stoichiometric exhaust gases, but also the rich-burned exhaust gases which are produced as a result of a fuel-rich air-fuel mixture and which has a lower oxygen partial pressure than the stoichiometric exhaust gases. The rich-burned exhaust gases contain a large amount of unburned components.
Thus, the A/F ratio of an air-fuel mixture can be determined by detecting a pump current (Ip) across the pair of pumping electrodes, which is varied with the oxygen concentration of the exhaust gases produced from the air-fuel mixture. The pump current (Ip) is expressed by the following equation: EQU Ip=A (Pe-Po)/R
where,
Pe=oxygen partial pressure of the exhaust gases PA0 Po=oxygen partial pressure of the atmosphere within the gas-diffusion space 4 PA0 R=resistance to diffusion of the exhaust gases from the external space into the gas-diffusion space 4 PA0 A=proportion constant PA0 .sigma..sub.0 : resistance to diffusion of exhaust gases through the gas-diffusion space PA0 Po.sub.2 ehx: oxygen concentration of exhaust gases in external space PA0 Po.sup.2 v: oxygen concentration of atmosphere within gas-diffusion space PA0 e: electric charge PA0 D: diffusion constant of oxygen PA0 A: cross sectional area of gas-diffusion space PA0 k: Boltzmann's constant PA0 T: absolute temperature PA0 l: length of diffusion through gas-diffusion space PA0 Vx: potential at a point upstream of the resistor PA0 Vo: potential at a point downstream of the resistor PA0 Rx: resistance to diffusion of the measurement gas into the first portion of the space 28 PA0 Ro: resistance to diffusion of the measurement gas into the second portion of the space 28 PA0 V (%): oxygen concentration of the measurement gas (exhaust gases) in the external space PA0 Vo (%): oxygen concentration of the atmosphere in the first or second portion of the space 28, which is established by the operation of the first or second pumping cell PA0 v: oxygen concentration of the measurement gas (exhaust gases) measured by the sensor which has served a given period of time PA0 Ip1': pump current of the first pumping cell PA0 Ip2': pump current of the second pumping cell PA0 Rx': diffusion resistance corresponding to the first portion of the space 28, after the given period of service of the sensor
Thus, the pump current (Ip) is influenced by the diffusion resistance (R) of the sensing element. The diffusion resistance (R) is changed during use of the A/F-ratio sensor, if the geometrical configuration of the diffusion path of the exhaust gases (measurement gas) is changed due to deposition of particles contained in the exhaust gases. Accordingly, the output of the A/F-ratio sensor in the form of the pump current (Ip) in relation to the excess air factor (A/F ratio) is changed from the state indicated in solid line in FIG. 3, to the state indicated in dashed line. Hence, the sensor suffers from a chronological change in its output characteristic, i.e., relationship between the pump current (Ip) and the A/F ratio.